JavaScript Regular Expressions

What you will practice

• Create a regular expression.
• Find matches in a string.
• Use test for simple validation.

What this means

A regular expression, often shortened to regex, is a pattern for matching text.

Regex is useful when plain string methods are too limited. You can search for emails, IDs, dates, repeated spaces, file extensions, or words with a certain shape.

Regex can become hard to read quickly. Small patterns with clear names are usually better than one giant clever pattern.

Mini glossary

Pattern

The rule a regular expression uses to match text.

Flag

A modifier such as `g` that changes how the regex searches.

test

A regex method that returns true or false.

How it works step by step

1. The code stores text that contains two email addresses.
2. The regex looks for a simple email-shaped pattern.
3. `match` returns all matches because the regex uses the `g` flag.
4. `test` checks whether a separate value looks like an email.

Where you will use this

• A signup form checks whether an email field has an email-like shape.
• A log tool extracts request IDs or error codes from pasted logs.
• A text cleanup tool finds repeated whitespace or unwanted characters.

Before you run the code

Regex validation is not the same as perfect truth. A simple email regex can catch obvious mistakes, but real email rules are more complicated.

Use regex as a practical text tool, and keep important business validation on the server too.

Common beginner mistakes

• Writing a pattern that is too broad and matches unwanted text.
• Writing a pattern that is too strict and rejects valid input.
• Forgetting the `g` flag when you want all matches, not only the first one.

Try changing this next

• Add another email address to the text.
• Remove the `g` flag and see how the result changes.
• Change the candidate to invalid text and check the boolean output.